Solar selective coatings are used as absorbers for harnessing solar energy for various applications. One of the essential requirements of solar selective absorbers is their stable structural composition when they operate at high temperatures. Optical properties of these coatings should not degrade with rise in temperature or over a period of use. The main utility of the present invention is for high temperature applications, particularly, in solar steam generators and steam turbines for producing the electricity.
In recent years a greater attention has been shown in harnessing alternative sources of energy like solar energy for industrial applications. Generally, concentrating type solar collectors are popularly used in industries for high temperature applications. Solar selective coatings applied to solar absorbers have been proved as an efficient method for harnessing the solar energy on large scale. The sputtering processes are widely being used to deposit solar absorber coatings for high temperature applications as these processes are environmental friendly and also offer to deposit complex compounds with controlled composition and microstructure. The coating of the present invention is deposited using a sputtering method, which is environmental friendly.
Earlier, applicant had developed a high temperature thermally stable solar selective coating, for effectively harnessing the solar energy. Patent application was filed and U.S. Pat. No. 7,585,568 was granted by USPTO for the invention. In U.S. Pat. No. 7,585,568, TiAlN/TiAlON/Si3N4 high temperature solar selective coating was deposited on various metal and non-metal flat substrates. Even though the invention has a great potential for solar thermal power generation, up-scaling of the process for industrial applications has following constraints: (1) It uses two separate sputtering systems to deposit the absorber layers (TiAlN and TiAlON) and anti-reflection layer (Si3N4). (2) It uses a composite TiAl target, therefore, the composition (i.e., contents of Ti and Al) of the absorber layer cannot be controlled independently. (3) The invention uses silicon (Si) as one of the source materials, which is expensive and very difficult to manufacture for large industrial sputtering machine. (4) The invention uses top-down geometry of the sputtering process and deposition on non-planar substrates is not feasible. (5) Long term thermal stability studies of the absorber coating under cyclic heating conditions and other aging tests have not been carried out.
All above limitations of the earlier invention directed inventors to evolve a coating formulation and deposition process, which can be suitable for high temperature applications in harnessing the solar energy. Present invention provides a multilayer solar selective coating containing tandem stacks of Ti/Chrome interlayer, aluminum-titanium nitride (AlTiN), aluminum-titanium oxynitride (AlTiON) and aluminum-titanium oxide (AlTiO). The solar selective coating of the present invention has been deposited by a single four-cathode reactive unbalanced pulsed direct current magnetron sputtering technique.
Prior-art search was made in public domain for patent as well as non-patent literature to find out the related work carried out in areas of the present invention. Some of the recent works, which are related to the field of the present invention, are discussed below.
A large number of solar selective coatings such as Ni—Al2O3, Ni—SiO2, Fe—Al2O3, Cr—SiO, Mo—Al2O3, Mo—SiO2, W—Al2O3, etc. have been developed for high temperature solar thermal applications. But only a few of them such as Mo—SiO2, W—Al2O3, Mo—Al2O3 and M—AlN (M: SS, W and Mo) cermets have been successfully commercialized and are being used in evacuated receiver tubes for solar thermal power generation. Mo—Al2O3 cermet coatings have been used on receiver tubes due to their excellent thermal stability in vacuum [Proceedings of the Society of Photo-Optical Instrumentation Engineers 1272 (1990) 240]. These receiver tubes were produced by Luz International Ltd., USA and were used in Solar Energy Generating System power plants. The Mo—Al2O3 cermet coatings were deposited using planar magnetron sputtering technology consisting of seven planar targets (three metallic and four ceramic targets), wherein, the metal targets are direct current (DC) sputtered and ceramic targets are sputtered using radio frequency (RF) power. Use of RF power supplies for industrial applications makes the process very expensive as well as cumbersome as a suitable matching network is required to operate an RF power supply. Additionally, sputtering of compound targets such as Al2O3 is challenging because of very low sputtering yield, high RF power levels and also control of stoichiometry of the deposited coating is extremely difficult. The Mo—Al2O3 cermet coatings are reported to exhibit an absorptance of 0.96 and emittance of 0.16 at 350° C. with thermal stability of 350-500° C. in vacuum. Despite the fact that this absorber coating is highly stable in vacuum, it has limited thermal stability in air (up to 300° C.).
It has been reported that the Mo—Al2O3 coatings are expensive when compared to other DC sputtered SS-C and AlN solar selective coatings, which are also produced on a commercial scale [Solar Energy 32 (1984) 609]. Double layer cermet concept has been developed to deposit SS-AlN coatings [Journal of Vacuum Science and Technology A 15 (1997) 2842] and these coatings are commercially marketed by TurboSun, China. The W—AlN and Mo—AlN double layer cermet coatings have been developed by sputtering process [U.S. Pat. No. 5,523,132, 1996, Journal of Physics D: Applied Physics 31 (1998) 355]. A solar absorptance of 0.92-0.94 and emittance of 0.08-0.10 at 350° C. were achieved for the W—AlN and Mo—AlN cermet coatings. These coatings are thermally stable at 350-500° C. in vacuum and are lower in cost than the Siemens CSP Tubes, Germany (formerly Solel Tubes) [Solar Energy Material and Solar Cells 62 (2000) 63]. Solel's Universal Vacuum Air Collector (UVAC2008) receiver tube uses an Al2O3 based multilayer cermet, which has an absorptance of 0.97-0.98 and emittance of 0.07-0.10 at 400° C. Further details about the substrate material and coating composition and properties are not available in the public domain.
Archimedes Solar Energy, Italy produces receiver tubes (HEMS08) for Italian National Agency for New Technologies, Energy and Environment (ENEA) Solar Thermodynamic Project, where the thermal exchange fluid is a molten salt entering at 290° C. in the solar field and coming out at 550° C. [http://www.archimedesolarenergy.com/receiver_tube.htm]. The receiver tube and the solar selective coating are reported to be very stable up to 580° C. The HEMS08 receiver tubes are coated with selective coatings of Mo—SiO2 (or) W—Al2O3 [Thin Solid Films 517 (2009) 6000, WO2009/107157 A2]. Solar absorptance greater than 0.94 and emittance lower than 0.13 (at 580° C.) have been reported for Mo—SiO2 coatings. The structure of this coating is as follows: Mo/Mo—SiO2 (HMVF)/Mo—SiO2 (LMVF)/SiO2. Similarly, graded W—Al2O3 coating exhibit α/∈(550° C.)=0.93/0.14 and this coating was thermally stable at 580° C. in vacuum, where HMVF and LMVF represent high metal volume fraction and low metal volume fraction, respectively. These inventors have also developed graded TiN—AlN cermets with AlN or Al2O3 antireflection coating exhibiting absorptance of 0.95 and emittance of 0.12 at 580° C. [WO 2005/121389 A1, 2005].
The composition of the PTR® 70 receiver tube developed by Schott, Germany is not known, but uses a new type of anti-reflection coating, which has a high abrasion resistance and at the same time allows the transmission of more than 96% of the sun's radiation [http://www.schottsolar.com/global/products/concentrated-solar-power/schott-ptr-70-receiver/]. The absorber coating has an absorptance of 0.95 and low emittance (<0.10) at a temperature of about 350-400° C. Further details about the absorber coating composition are not available in the public domain.
References may be made to Surface and Coatings Technology: [163-164 (2003) 674], [200 (2006) 6840], [201 (2007) 6699] and [204 (2009) 256], wherein various researchers have developed nanocrystalline AlTiN coatings for dry and high speed machining of hardened tool steel. The AlTiN coating has been shown to exhibit extraordinary performance in high speed machining of hardened tool steel. This is attributed to high adhesion, ultra-fine crystalline as well as high oxidation resistance of the coating. The high oxidation resistance of nanocrystalline AlTiN coating has been related to the formation of aluminum oxide (Al2O3) surface layer. The oxide formation has been shown to be more pronounced for nanocrystalline coating as it promotes rapid diffusion of Al to the surface along the grain boundaries.
References may also be made to Materials Science and Engineering A 528 (2011) 4703, wherein researchers have used AlTiON coatings for protection against oxidation of hot work tool samples. It has been reported that the formation of Al2O3 at elevated temperatures improves the performance of the coated tools. Similarly, references may also be made to Thin Solid Films 515 (2006) 346, wherein AlTiO films have been developed on silicon substrates for metal-oxide-semiconductor (MOS) devices. The AlTiO films exhibit very high dielectric constant, twice as large as demonstrated by the well known HfAlO dielectric thin films. The search on public domain regarding optical properties of AlTiN, AlTiON and AlTiO yielded no results.
References may be made to “Preparation and thermal stability of non-vacuum high temperature solar selective absorber coatings” [Chinese Science Bulletin 54 (2009) 1451] and “Non-vacuum solar spectrum selective absorption film and preparation method thereof” [Chinese Patent: CN 101666557A], wherein approximately 2.0 μm thick TiAl/TiAlN/TiAlNO/TiAlO absorber layer has been prepared using a multi-arc ion deposition facility from a TiAl alloy target with ratio of Ti to Al of 50:50. These inventors have reported that the said coating exhibits high absorptance (0.90) and low emittance (0.09-0.19) and remains stable in air up to 650° C. for 1 hr. Long term thermal stability in air and vacuum and detailed studies on the optical properties have not been reported by these inventors. The process employed in this invention uses multi-arc ion plating, which has inherent disadvantage that dense and uniform absorber coating with optical thicknesses (λ/4≅120 nm) cannot be prepared. Also the invention uses a TiAl alloy target and it is not possible to control the content of Ti and Al in the absorber layers independently. Additionally, the multi-arc ion plating process introduces a large number of metal droplets, which deteriorate the properties of the deposited coatings.
References may also be made to “High temperature solar selective coating” [U.S. Patent No. 2010/0313875 A1], wherein the absorber tubes are coated with improved solar selective coating comprising of several layers of refractory metals or metalloid oxides (titania and silica) with substantially differing indices of refraction in adjacent layers. The absorber layers include cermets materials comprising particles of metal compounds in a matrix, which contain oxides of refractory metals or metalloids such as Si. At least one layer of Pt is also included between some of the absorber layers. The absorber coating also comprises reflective layers from the following compounds: TiSi, Ti3SiC2, TiAlSi, TiAlN, Ti3O5, TiOx or TiOxN1-x, etc. These multilayer absorber coatings have been found to have a stable thermal emittance up to 500° C.
In order to manufacture the absorber coatings for industrial applications it is important that the deposition process should be simpler and involve less processing steps and also the raw materials should be cost effective, yet the absorber coating must exhibit high thermal stability and high solar selectivity. None of the prior-art referred as above shows all these features. Therefore, there is a need to develop easy to process and cost effective high temperature solar selective coatings for solar thermal power generation applications.
The present invention also allows deposition of all the layers in a single sputtering chamber, thus making the process simpler and cost effective. The present invention is capable of depositing absorber coating on both planar and tube-like substrates. The tubular substrates with a length of approximately 140 mm and diameter up to 100 mm can be coated in the present invention. The process of the present invention can be up-scaled easily for the deposition on longer tubes with good uniformity and reproducibility, considering the above limitations as disclosed in the prior-art literature.